The proposed work is to continue studies on the structure and genetics of the avian immune system with emphasis given to: (1) the structure of chicken allotypes; (2) the mechanism for expression of latent (unexpected) allotypes; (3) the fine structure of idiotypes; (4) development of congenic chicken lines with hopes of mapping MHC genes; (5) the relation of immune responsiveness to related polymers to the MHC; (6) delineation of T lymphocyte subsets with respect to function and surface markers.